1. Field of the Invention
The present invention relates generally to a liquid crystal display device and more particularly to an active matrix liquid crystal display device.
2. Description of the Related Art
In general, a liquid crystal display device includes a liquid crystal display panel having a display section on which an image is displayed. The liquid crystal display panel includes a pair of mutually opposed substrates. A liquid crystal layer is held between the pair of substrates.
One of the substrates includes a plurality of pixel electrodes which are arrayed substantially in a matrix. The other substrate includes a counter-electrode which is opposed to the plural pixel electrodes. A pair of alignment films for aligning a liquid crystal are disposed on the plural pixel electrodes and the counter-electrode. The alignment state of the liquid crystal is obtained by controlling the alignment of liquid crystal molecules on the pair of alignment films. A rubbing method, for instance, is known as a method of controlling the alignment of liquid crystal molecules. In the rubbing method, the alignment film is rubbed by a rubbing cloth. Thus, an average direction of major axes of liquid crystal molecules is controlled by the rubbing treatment.
In recent years, attention has been paid to a so-called black insertion driving method in which black signal write and video signal write are repeated in one frame period, thereby to enhance the video quality in the liquid crystal display panel and to improve the response speed. In the operation of the liquid crystal display panel to which this driving method is applied, the alignment state of liquid crystal molecules is cyclically varied by cyclical application of a black signal and a video signal. Thereby, a flow occurs in the liquid crystal layer substantially in the same direction as the rubbing direction of the alignment film.
In addition, from the standpoint of high responsivity and a wide viewing angle, attention has been paid to an OCB (Optically-Compensated-Bent) mode as a display scheme of a liquid crystal display device. In the OCB mode liquid crystal display device, such a driving method is applied that display is effected on the basis of a reverse transition prevention signal and a video signal in one frame, thereby to improve the video quality. If this driving method is applied, a flow occurs due to a variation in alignment of liquid crystal molecules in the liquid crystal layer substantially in the same direction as the rubbing direction of the alignment film.
There is a case in which ions are taken in during the fabrication process of the liquid crystal display device, or the material itself of, e.g. glass substrates, which constitute the liquid crystal display device, contain ions. Such ions move in the liquid crystal layer in the display section due to the flow occurring in the liquid crystal layer. On the other hand, no flow occurs in the liquid crystal layer in the peripheral section. As a result, ions may agglomerate in the vicinity of a boundary between the display section and the peripheral section. In the region of the liquid crystal layer where ions agglomerate, the transmittance versus application voltage characteristics vary, and such a variation, in some cases, may be recognized as display non-uniformity.
In the prior art, there has been proposed an invention of an OCB mode liquid crystal display device wherein impurity ions are collected on ion trap electrodes provided at the peripheral section surrounding the display section, thereby to solve the problem that spacers, such as beads, or impurity ions move by the flow occurring due to the variation in alignment of liquid crystal molecules, and to prevent occurrence of a display defect such as display non-uniformity (see Jpn. Pat. Appln. KOKAI Publication No. H9-54325).
In the above-described prior art invention, however, no consideration is given to the rubbing direction of alignment films, and it is difficult to effectively suppress a display defect due to ions which move in the rubbing direction. In addition, in the peripheral section, if a fixed DC voltage, with which the liquid crystal molecules are bend-aligned, is applied to the liquid crystal layer, the viscosity of the liquid crystal layer increases. Hence, diffusion of the liquid crystal layer does not easily occur in the peripheral section, and impurity ions, which move in one direction due to the flow of the liquid crystal layer, do not move to the peripheral section and may agglomerate in the vicinity of the boundary between the display section and the peripheral section.
In particular, in a liquid crystal display panel with a small picture-frame region surrounding the display section, the picture-frame region is small, compared to the amount of ions in the entire liquid crystal display panel, thus leading to a case in which ions spreads into the display region due to long-time driving.
As has been described above, in a region where agglomeration of ions occurs, such as a region on a terminal end side in the rubbing direction, the transmittance versus application voltage characteristics of the liquid crystal layer vary due to ions. In particular, the transmittance versus application voltage characteristics of the liquid crystal layer vary in the display section, and such a variation, in some cases, may be recognized as display non-uniformity.